Ruby example of using Open3.popen3 with a select loop to read a child process's standard output and standard error streams.

popen3_test

#!/usr/bin/env ruby

require 'open3'

# Returns true if all files are EOF
#
def all_eof(files)
  files.find { |f| !f.eof }.nil?
end

command = "ls -l"

puts "Running command: #{command}"

BLOCK_SIZE = 1024

Open3.popen3(command) do |stdin, stdout, stderr|
  stdin.close_write

  begin
    files = [stdout, stderr]

    until all_eof(files) do
      ready = IO.select(files)

      if ready
        readable = ready[0]
        # writable = ready[1]
        # exceptions = ready[2]

        readable.each do |f|
          fileno = f.fileno

          begin
            data = f.read_nonblock(BLOCK_SIZE)

            # Do something with the data...
            puts "fileno: #{fileno}, data: #{data}"
          rescue EOFError => e
            puts "fileno: #{fileno} EOF"
          end
        end
      end
    end
  rescue IOError => e
    puts "IOError: #{e}"
  end
end

puts "Done"

mumoshu commented on Nov 18, 2015:

Hi, thanks for sharing this code

Beware that IO#eof blocks the current thread until the IO is ready to read(which you may wanted to handle by IO.select).
As result:

At line 23 of your code, all_eof(files) blocks when stderr is flowing but stdout is stopped. stderr is never read until stdout is ready.
ready = IO.select(files) always return immediately(because at line 23 of your code, you already waited for all the files to be read-ready)
I guess what you may want to do is:

until files.empty? do # modified
  ready = IO.select(files)

  if ready
    readable = ready[0]

    readable.each do |f|
      fileno = f.fileno

      begin

        data = f.read_nonblock(BLOCK_SIZE)

        # Do something with the data...
        puts "fileno: #{fileno}, data: #{data}"
      rescue EOFError => e
        puts "fileno: #{fileno} EOF"
        files.delete f # added
      end
# *snip*

Btw, I've managed to write a wrapper command(wrapper cat acts the same as cat) thanks to your code :)
https://github.com/crowdworks/joumae-ruby/blob/master/lib/joumae/command.rb#L32